To meet governmental requirements during plugging and abandonment (P&A) operations in a well, a deep set barrier must be installed as close to the potential source of inflow as possible, covering all leak paths. A permanent well barrier shall extend across the full cross section area of the well, including all annuli, and seal both vertically and horizontally in the well. This requires removal of tubing mechanically, or perforating tubulars followed by washing behind the tubulars. This will lead to that swarf and debris from for example mechanical milling, need to be cleaned out of all flowlines, including the BOP system, to the rig. Normally cement is used for the purpose of P&A operations. However, the well barrier has to comply with all of the following requirements for a P&A plug; a) impermeability, b) long term integrity, c) non shrinking, d) ductility (non brittle)—able to withstand mechanical loads or impact, e) resistance to different chemicals/substances (H2S, CO2 and hydrocarbons) and f) wetting—to ensure bonding to steel.
The applicant has invented an alternative method of performing P&A operations, using a heat generating mixture, e.g. a thermite mixture. Thermite is normally known as a pyrotechnic composition of a metal powder and a metal oxide. The metal powder and the metal oxide produce an exothermic oxidation-reduction reaction known as a thermite reaction. A number of metals can be the reducing agent, e.g. aluminium. If aluminium is the reducing agent, the reaction is called an aluminothermic reaction. Most of the varieties are not explosive, but may create short bursts of extremely high temperatures focused on a very small area for a short period of time. The temperatures may reach as high as 3000° C.
There exist prior art solutions where thermite is used within the field of well technology. Examples are disclosed in documents US 2006/144591 A1 (Gonzalez et al.) and U.S. Pat. No. 6,923,263 B2 (Eden et al.). US 2006/144591 A1 describes the use of molten metal plugs in wells. The object of US 2006/144591 A1 is to melt a meltable repair material, such as an eutectic material, utilizing an exothermic reactant material. The method disclosed comprises introducing a meltable repair material proximate a structure in a subterranean well where a fluid seal is desired. Exothermic reactant materials are located proximate the meltable repair material. The exothermic reactant is ignited or otherwise initiated to create an exothermic reaction which supplies heat to and melts the meltable repair material into a molten mass. The molten mass flows and solidifies across the structure and the fluid seal defect to effect a fluid seal in the subterranean well structure. Suitable exothermic reactant materials exemplified includes thermite, thermate and highly exothermic chemical reactions such as the reaction between ammonium chloride and sodium nitrite, while preferred meltable materials include solder and eutectic metals which expand upon cooling and solidifying from a molten state.
U.S. Pat. No. 6,923,263 B2 discloses an apparatus for forming a plug in a casing including a body of plug material and a carrier for insertion into a casing. The carrier supports the body of plug material. The carrier includes a mandrel and at least two circular flanges spaced apart along the mandrel. The carrier also includes a heater for heating the mandrel. The mandrel is heated to a temperature above the melting point of the material in the mandrel and the plug material slumps into the at least two circular flanges. The at least two circular flanges force the expanded solidifying plug against the casing which aids the transfer of heat between the mandrel and the plug material, and resists creep of solidified material along the casing.
Other prior art solutions are known from documents US 2002/170713 A1 and U.S. Pat. No. 4,298,063 A.
A common feature in the disclosed prior art solutions is that the metal plug material is introduced in to the well. Additionally, the plug is formed substantially on the inside of a tubular, such as a casing or tubing, forming a metal seal on the inside of said tubular. Therefore, the melting point of the introduced plug material has to be lower than the melting point of the surrounding tubular to avoid that the surrounding tubular melts.
An object of the invention is to provide a method for permanent well abandonment or removal of a well element arranged in a well by the use of a thermite mixture.
Another object of the invention is to reduce or remove the need for a rig in P&A operations.